High performance, flaw-free coatings of electrical insulation are highly desirable for applications in the aerospace and other industries. One route to such coatings is by electrophoretic deposition whereby a resin migrates in an electric field to a charged workpiece, depositing a coating thereon. Because the resin seeks out areas of greatest charge density, the resulting coatings tend to be pinhole free.
The high performance resins which are most suited to electrophoretic deposition, are those containing aromatic polyamic acid linkages. When cured, these resins condense out water, forming a polyimide. Until now, it was believed that polyamic acid polymers could not be electrophoretically deposited unless they were in the form of a salt. A salt was produced by reacting the free carboxyl group of the polyamic acid with an amine or a surface active agent. The charge on the salt was thought to be necessary to cause the polyamic acid to migrate to the oppositely charged workpiece. Because amines are basic, previous electrodeposition of polyamic acids were conducted in an emulsion having an apparent pH of about 8 to about 10.